1. Field of the Invention
The present invention relates to a photoresist material which is suitably used for the manufacure of a semiconductor element and, more particularly, to a photoresist material which has a high glass transition point, a high sensitivity to radiation, and an excellent dry etching resistance.
Various industrial techniques have been conventionally proposed which utilize photosensitive compositions. For example, in the lithography process in photomechanical processes for the manufacture of semiconductor elements, photosensitive compositions for various purposes have been proposed and have been actually used. Most of such compositions are called photoresists; parts of such photoresists which are irradiated with light, ultraviolet rays or the like respond thereto such that the irradiated parts of the photoresists have a different solubility from that of the non-irradiated parts thereof, thereby forming a pattern.
In the field of manufacuture of semiconductor elements, highly integrated elements such as a VLSI, a magnetic bubble element or the like can now be manufactured with recent fast technical developements. However, with the conventional photolithography utilizing light or ultraviolet rays having a wavelength of about 400 nm, the resolution obtainable is limited. For this reason, lithography utilizing radiation with a shorter wavelength such as far ultraviolet rays, X-rays, an ion beam or an electron beam are beginning to be used.
A resist material to be used in lithography utilizing far ultraviolet rays, X-rays, an electron beam or the like must have high sensitivity to such radiation and must also having a high dry etching resistance in order to provide a high resolution. A composition having aromatic rings is generally known to have a high dry etching resistance. As resist materials having aromatic rings have been proposed, for example, polystyrene, poly-.alpha.-methylstyrene, halogenated polystyrene, chloromethylated polystyrene and the like, which exhibit excellent dry etching resistance.
However, these resist materials have a low glass transition point Tg; 105.degree. C. for polystyrene and so on. For this reason, special care must be taken for temperature rise during dry etching. Although the speed of dry etching is generally increased with an increase in the substrate temperature and an increase in the etching power, a high etching power results in a high surface temperature of the substrate. If the temperature of a resist layer formed on the surface of the substrate exceeds its Tg, the dry etching resistance is known to abruptly decrease. Accordingly, etching must be performed while cooling the substrate. In this manner, if the dry etching speed is increased, a temperature rise in the resist layer cannot be avoided. In view of this problem, the temperature rise of the substrate must be properly controlled, even if at the cost of throughput. Resist materials having a higher Tg have thus been desired. After printing a pattern on a resist, it is generally post-baked. The baking temperature is set in accordance with each application. For example, if the adhesion strength between the substrate and the resist must be improved, the baking temperature is desirably high within a range such that the pattern may not thermally deform. With a resist material having a low Tg, the baking temperature must also be lowered accordingly. Thus, in order to improve the adhesion strength between a substrate and a resist, a resist material having a high Tg, that is, a resist material having a high resistance to heat and a high sensitivity, is desired.
Among the resist materials having aromatic rings mentioned above, the poly-.alpha.-methylstyrene has a glass transition point of 192.degree. C. which is relatively high. On the other hand, the sensitivity is low, and a considerably high sensitivity may not be obtained even if chloromethyl groups which are highly sensitive groups are introduced.